1. Field of the Invention
The present invention relates to a process for preparing binders which contain allophanate groups, groups that react with ethylenically unsaturated compounds with polymerization on exposure to actinic radiation and which optionally also contain isocyanate-reactive groups, to the resulting binders and to their use in coating compositions.
2. Description of Related Art
The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology. Coating compositions based on this principle are thus referred to as radiation- or actinically curing or curable systems.
Particularly advantageous properties can be obtained if the radiation cure is combined with a second crosslinking step so that it can be controlled independently therefrom. Coating systems of this kind are referred to as dual-cure systems (e.g. Macromol. Symp. 187, 531-542, 2002, defined on p. 534).
Because of the environmental and economic requirements imposed on modern coating systems, i.e., that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose are polyisocyanates having allophanate groups as described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residue isocyanate content sufficiently (<0.5% by weight of residual monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no radiation-curing derivatives with activated double bonds are known. All that is mentioned is the use of maleinate- and/or fumarate-containing polyesters; the possibility of radiation curing is not described.
U.S. Pat. No. 5,777,024 describes the preparation of radiation-curing allophanates of low viscosity by a reaction of hydroxy-functional monomers that carry activated double bonds with allophanate-modifier isocyanurates that contain NCO groups.
The formation of allophanate compounds by ring opening of uretdiones with alcohols is known in principle as a crosslinking mechanism in powder coating materials (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also US-A 2003/0153713). Nevertheless, the reaction temperatures required for this purpose are too high (≧130° C.) for a targeted preparation of radiation-curing monomers based on allophanate with activated double bonds.
Historically the direct reaction of uretdione rings with alcohols to allophanates was first investigated for solventborne, isocyanate-free, 2K [2-component] polyurethane coating materials. Without catalysis this reaction is of no technical importance, due to the low reaction rate (F. Schmitt, Angew. Makromol. Chem. (1989), 171, pp. 21-38). With appropriate catalysts, however, the crosslinking reaction between HDI-based uretdione curatives and polyols is said to begin at 60 to 80° C. (K. B. Chandalia; R. A Englebach; S. L. Goldstein; R. W. Good; S. H. Harris; M. J. Morgan; P. J. Whitman; R. T. Wojcik, Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium, (2001), pp. 77-89). The structure of these catalysts has not been published to date. Commercial products prepared by utilizing this reaction are also undisclosed to date.
In summary it may be stated that the preparation of radiation-curing allophanates of low viscosity having isocyanate-reactive groups by a ring-opening reaction of alcohols that carry activated double bonds with uretdiones at temperatures ≦130° C. is not explicitly described by the prior art.
Surprisingly it has now been found that from the reaction of uretdiones with olefinic unsaturated alcohols that preferably contain activated double bonds and saturated compounds having at least two isocyanate-reactive groups it is possible, using ammonium salts or phosphonium salts of aliphatic carboxylic acids as catalysts, to obtain radiation-curing allophanates of low viscosity with low residual monomer fractions at temperatures even of ≦130° C. When such crosslinkers contain not only radiation-curing functions but also functions that are reactive towards NCO groups, they are referred to as dual-cure crosslinkers.